The present invention relates generally to a system and method for optimizing riserless drilling casing seats used in offshore deepwater drilling from a floating platform. More particularly, the present invention uses a system and method for determining the optimal placement of the initial casing seats by using, among other criteria, the relationship between the pore pressures and fracture pressures to determine a depth that will optimize placement of riserless casing seats to achieve deeper well depths, minimize casing diameter reduction, decrease the likelihood of well failure and more efficient use of well construction materials.
The continuing demand for crude oil and natural gas combined with the limited number of near shore fields and has provoked the exploration and production of offshore crude oil and natural gas to increasing water depths. Increasing water depths have required the use of floating platforms that support a drilling rig and drilling equipment. Advances in floating platform technology has increased the weight loads that the platforms can safely utilize and as such, drilling strings, generally formed of jointed steel pipe, can reach greater depths.
In conventional floating platform deepwater drilling, riserless drilling is used. In riserless drilling there is no return conduit provided back to the platform surface, as is done in many shallow water drilling operations. In conventional riserless drilling, the drilling cuttings and other by-products are discharged to the seafloor and are typically swept away by currents. In drilling the riserless portion of the well, the first casing, typically of a length of about 250 to 350 feet, is lowered from the platform and jetted into place into the seafloor. This first string of casing is commonly referred to as the structural or conductor string. A general description of riserless drilling is provided in U.S. Pat. No. 7,150,324, the entire substance of which is incorporated herein by reference.
The current approach of “jetting” in the first string of casing, usually 250 to 350 ft below the mud line, results in a casing seat being placed too shallow thereby not providing enough leak-off tolerance for the drilling of the next hole section. This is due to the very soft formations which have little strength or competency for fracture resistance and leak-off. The current philosophy of the first casing seat placement is to provide structural support for the weight of the subsequent casing strings and the bending moment of the riser, which will be eventually attached. The general intended purpose of the structural string is limited to supporting the weight of subsequent casing strings and wellhead, and the resistance of bending moment of the riser loading. Despite this perception, in reality, the structural string's ability to support much of an axial load is limited and thus can become a structural failure hazard if there is not enough soil bearing strength for the landing of the subsequent strength of casing and wellhead. The conventional approach adds little to the value of the well design, since this casing setting depth does not supply sufficient axial loading resistance for structural support of subsequent deeper casing strings nor does it supply sufficient bending load or sufficient rising bending moment. Also, there is no value related to the growth of the fracture gradient in the first string and that negatively impacts the overall well design by wasting casing diameters. Because the conventional placement of the casing well above every anticipated drilling hazard, such placement negatively impacts the casing diameters and hole sizes for well depths that routinely exceed 30,000 ft in measured depth. In this regard, the structural casing placement has been conventionally completed without regard to its optimal placement depth, but rather as a mere first step in the process of riserless drilling.
As is understood in the art, deepwater oil drilling is an expensive and time intensive venture. Daily operating costs often approach $1,000,000.00 requiring 100 days or more to drill before achieving the well objectives. Therefore, it is critical to deepwater field development to reduce well costs and to improve the attainment of these well objectives. The complex deepwater drilling environments have pushed well design to its limits and while many of the aspects of deepwater drilling and well design are being optimized, the optimal placement of the first and subsequent casing seats have been overlooked. As such there is a need in the art for a system and method that takes advantage of the increased maximum loads from floating platforms and provides for the determination of the optimal depth of placement of the early depth casing seats and placement of those seats to maximize drilling depths, drilling time and costs of operation.